In a single-shaft system for generating electrical energy, a gas turbine, a steam turbine and a generator are arranged on a common train. During normal operation of the single-shaft system, the electrical energy is fed into an electrical grid and the train rotates at a speed which corresponds to the rated speed of the single-shaft system, for example 50 Hz or 60 Hz. In the event of a fault, in particular in the event of a drop in the electric load connected to the generator, the speed can rise to values above the rated speed. If the speed reaches a critical speed, the single-shaft system is subjected to excessive mechanical and thermal load, resulting in a reduction in the service life of the single-shaft system.
When a limit speed is reached, an overspeed protection apparatus, which prevents a further rise in the speed of the train, engages, wherein the limit speed is conventionally chosen so as to lie between the rated speed and the critical speed. Conventionally, the overspeed protection apparatus is tested by operating the single-shaft system in test operation, in which the limit speed is reduced with respect to the limit speed in normal operation, in order to thus avoid excessive load on the single-shaft system during test operation.
However, it would be desirable to use, in test operation, the same limit speed as in normal operation. In addition, such a test is compulsory in certain countries, for example in South Korea.